Question : OS Logs On and then Immediately Logs Off

I am working on a client's XP Pro computer that was infected and also would log in and immediately log out. I cleaned it (as a slave) using AVG, Superantivirus and MBAM (several infections were found) but it still logs in and immediately logs off.
I can access F8 (Safe Mode also logs on and then off) and use the CD ROM. I can also work on the HDD files using an IDE\Sata adapter. I have the an XP Pro install disk.
I have researched (on this site and others) the log on log off issue and have found and/or tried the following:
1) All references seem to be several years old.
2) Some involve infected computers with a registry entry that points to "wsaupdater.exe" rather than the correct entry which should be "userinit.exe".
3) Some suggest replacing windows\system32\wsaupdater.exe with userinit.exe. I did not find "wsaupdater.exe" in my client's system32 folder.
4) One link lead me to Microsoft with the instructions to (thru the recovery console and the XP disk) type "cd system32" and "copy userinit.exe wsaupdater.exe" The file file was copied but the process did not work.
5) Some sites refer to a registry entry that should point to explorer.exe
It seems to me that there is a bad registry entry. As to whether it is "wsaupdater.exe" I do not know.
My questions are:
1) Does anybody have any, more recent, experience  with this log on then immediate log off issue and if so how was it solved?
2) Is there any software out there that provides read, write, search capabilities for a registry on an unworkable OS?
Thanks

Answer : OS Logs On and then Immediately Logs Off

Since you can work on the HDD files using an IDE\Sata adapter.

You can load the registry form the host computer.

Within regedit, Click on HKey_Local_Machine so its highlighted.

Click File, Load Hive,  Browse to windows\System32\Config and choose System,

Create a name like No Logon.

No Logon should now appear under My Computer.

Open HK-L-M : Sofwate, Microsft, Windows Nt , Current Version, Windows NT, Winlogon

There should be a valu label shell , that should be the explorer.exe

You can search and write to this hive and when you unload it the changes will get saved.

You might also want to try the Microsoft User Profile Hive Cleanup Service (UPHClean), See: http://support.microsoft.com/kb/837115

Also to clarify what I wrote above see this link although not exactly the issue you are having it gives some good direction:Black startup screen is briefly displayed and your computer restarts repeatedly when you try to start Windows XP :  http://support.microsoft.com/kb/314466

Wes